1. Technical Field
The present invention relates to a magnetic head having a confined current path, and more specifically, to a ballistic magneto resistive (BMR) sensor having a free layer stabilized by an in-stack bias and spacer-decoupling layer including nanoparticles.
2. Related Art
In the related art magnetic recording technology such as hard disk drives, a head is equipped with a reader and a writer that operate independently of one another. FIGS. 1 (a) and (b) illustrate related art magnetic recording schemes. A recording medium 1 having a plurality of bits 3 and a track width 5 has a magnetization 7 parallel to the plane of the recording media. As a result, a magnetic flux is generated at the boundaries between the bits 3. This is commonly referred to as “longitudinal magnetic recording”.
Information is written to the recording medium 1 by an inductive write element 9, and data is read from the recording medium 1 by a read element 11. Coils 16 are used to supply a write current 17 to the inductive write element 9, and a read current 15 is supplied to the read element 11. An insulating layer (not illustrated for the sake of clarity) made of Al2O3 or the like is deposited between the read element 11 and the write element 9 to avoid any interference between the respective read and write signals.
The read element 11 is a sensor that operates by sensing the resistance change as the sensor magnetization changes direction. A shield 13 reduces the undesirable magnetic fields coming from the media and prevents the undesired flux of adjacent bits from interfering with the one of the bits 3 that is currently being read by the read element 11.
Due to requirements of increased bit and track density readable at a higher efficiency and speed, the related art magnetic recording scheme of FIG. 1(b) has been developed. In this related art scheme, the direction of magnetization 19 of the recording medium 1 is perpendicular to the plane of the recording medium 1. This is also known as “perpendicular magnetic recording”. This design provides more compact and stable recorded data. Also a soft underlayer (not illustrated) is required to increase the writer magnetic field efficiency. Further, an intermediate layer (not illustrated for the sake of clarity) can be used to control the exchange coupling between the recording layer 1 and soft underlayer.
FIGS. 2(a)-(c) illustrate various related art read heads for the above-described magnetic recording scheme, known as “spin valves”. In the bottom type spin valve illustrated in FIG. 2(a), a free layer 21 operates as a read sensor to read the recorded data from the recording medium 1. A spacer 23 is positioned between the free layer 21 and a composed pinned layer 25. On the other side of the composed pinned layer 25, there is an anti-ferromagnetic (AFM) layer 27. In the top type spin valve illustrated in FIG. 2(b), the position of the layers is reversed.
FIG. 2(c) illustrates a related art dual type spin valve. Layers 21 through 25 are substantially the same as described above with respect to FIGS. 2(a)-(b). However, an additional spacer 29 is provided on the other side of the free layer 21, upon which a second pinned layer 31 and a second AFM layer 33 are positioned. An extra signal provided by the second pinned layer 31 increases the resistance change ΔR.
The direction of magnetization in the pinned layer 25 is substantially fixed, whereas the direction of magnetization in the free layer 21 can be changed, for example (but not by of limitation) depending on the effect of an external magnetic field, such as the recording medium 1.
When the external magnetic field is applied to a reader, the magnetization of the free layer 21 is altered, or rotated, by an angle. When the flux is positive the magnetization direction of the free layer 21 is rotated upward, and when the flux is negative the magnetization direction of the free layer 21 is rotated downward. If the applied external field changes the free layer 21 magnetization direction to be aligned in the same way as composed pinned layer 25, then the resistance between the layers is low, and electrons can more easily migrate between those layers 21, 25.
However, when the free layer 21 has a magnetization direction opposite to that of the composed pinned layer 25, the resistance between the layers is high. This high resistance occurs because it is more difficult for electrons to migrate between the layers 21, 25. Similar to the external field, the AFM layer 27 provides an exchange coupling and keeps the magnetization of composed pinned layer 25 substantially fixed.
The resistance change ΔR when the layers 21, 25 are parallel and anti-parallel should be high to have a highly sensitive reader. The media bit is decreasing in size, and the correspondingly, the magnetic field from the media bit is weaker. As a result, it is necessary for the free layer to sense this media flux having a reduced magnitude. Therefore, it is important for the related art free layer to have a reduced thickness to maintain sufficient sensitivity of the free layer. In order to provide a high-sensitivity sensor that can sense a very weak magnetic field, this is accomplished by reducing the free layer thickness to about 3 nm in the case of an areal recording density of 150 to 200 Gbits/in2.
However, as a result of the thin free layer, there is a related art problem of a stronger spin transfer effect. The spin transfer effect is substantially inversely proportional to the thickness of the film. Thus, the stability of the free layer is reduced. Further, there is also a need for a high resistance change ΔR between the layers 21, 25 of the related art read head. As discussed in greater detail below, a thicker free layer results in a higher value of ΔR.
The operation of the related art read head is now described in greater detail. In the recording media 1, flux is generated based on polarity of adjacent bits in the case of longitudinal magnetic recording. If two adjoining bits have negative polarity at their boundary the flux will be negative. On the other hand, if both of the bits have positive polarity at the boundary the flux will be positive. The magnitude of flux determines the angle of magnetization between the free layer and the pinned layer.
FIG. 3 illustrates a related art synthetic spin valve. The free layer 21, the spacer 23 and the AFM layer 27 are substantially the same as described above. However, the composed pinned layer 25 further includes a first pinned sublayer 35 separated from a second pinned sublayer 39 by a pinned layer spacer 37. The first pinned sublayer 35 operates according to the above-described principle with respect to the composed pinned layer 25. The second pinned sublayer 39 has an opposite spin state with respect to the first pinned sublayer 35. As a result, the total composed pinned layer magnetic moment is reduced due to anti-ferromagnetic coupling between the first pinned sublayer 35 and the second pinned sublayer 39. The synthetic read head has a composed pinned layer with a total magnetic flux close to zero, and thus greater stability and high pinning field can be achieved than with the single pinned layer structure. A buffer layer 28 is deposited below the AFM layer 27 for good spin-valve growth, and a cap 40 is provided on an upper surface of the free layer 21.
FIG. 4 illustrates the related art shielded read head. As noted above, it is important to avoid the sensing of unintended magnetic flux from adjacent bits during the reading of a given bit. A cap (protective) layer 40 is provided on an upper surface of the free layer 21 to protect the spin valve against oxidation before deposition of top shield 43, by electroplating in a separated system. Similarly, a bottom shield 45 is provided on a lower surface of the buffer layer 28.
Related art magnetic recording schemes use a current perpendicular to plane (CPP) head, where the sensing current flows perpendicular to the spin valve plane. As a result, the size of the read head can be reduced without a loss of the output read signal. Various related art spin valves that operate in the CPP scheme are illustrated in FIGS. 5(a)-(c), and are discussed in greater detail below. These spin-valves structurally differ primarily in the composition of their spacer 23. The compositions and resulting difference in operation of these effects is discussed in greater detail below.
FIG. 5(a) illustrates a related art tunneling magnetoresistive (TMR) head for the CPP scheme. In the TMR head, the spacer 23 acts as an insulator, or tunnel barrier layer. Thus, in the case of a very thin barrier that is the spacer 23, the electrons can migrate from free layer 21 to pinned layer 25 or verse versa without change of spin direction. Current related art TMR heads have an increased magnetoresistance (MR) on the order of about 30-50%.
FIG. 5(b) illustrates a related art CPP-GMR head. In this case, the spacer 23 acts as a conductor. In the related art CPP-GMR head, there is a need for a large resistance change ΔR, and a moderate element resistance for having a high frequency response. A low free layer coercivity is also required so that a small media field can be detected. The pinning field should also have a high strength. Additional details of the CPP-GMR head are discussed in greater detail below.
FIG. 5(c) illustrates the related art ballistic magnetoresistance (BMR) head. In the spacer 23, which operates as an insulator, a ferromagnetic region 47 connects the pinned layer 25 to the free layer 21. The area of contact is on the order of a few nanometers. This is referred to as a nano-path or a nano-contact. As a result, there is a substantially high MR, due to electrons scattering at the domain wall created within this nanocontact. Other factors include the spin polarization of the ferromagnets, and the structure of the domain that is in nano-contact with the BMR head.
In the foregoing related art heads, the spacer 23 of the spin valve is an insulator for TMR, a conductor for GMR, and an insulator having a magnetic nano-contact for BMR. While related art TMR spacers are generally made of insulating materials such as alumina, related art GMR spacers are generally made of conductive metals, such as copper.
In the related art GMR head, resistance is minimized when the magnetization directions (or spin states) of the free layer 21 and the pinned layer 25 are parallel and is maximized when the magnetization directions are opposite As noted above, the free layer 21 has a magnetization of which the direction can be changed. Thus, the GMR system avoids perturbation of the head output signal by minimizing the undesired switching of the pinned layer magnetization.
GMR depends on the degree of spin polarization of the pinned and free layers, and the angle between their respective magnetizations. Spin polarization depends on the difference between the spin state (up or down) in each of the free and pinned layers. As the free layer 21 receives the flux from the magnetic recording media, the free layer magnetization rotates by a small angle in one direction or the other, depending on the direction of flux. The change in resistance between the pinned layer 25 and the free layer 21 is proportional to angle between the moments of the free layer 21 and the pinned layer 25, as noted above. There is a relationship between the resistance change ΔR and the output read signal.
The GMR head has various requirements. For example, but not by way of limitation, a large resistance change ΔR is required to generate a high output signal. In order to generate the large resistance change ΔR, it is desirable to have thicker free layer. This relationship is shown in FIG. 6(a). A similar relationship exists between the MR ratio and free layer thickness, as shown in FIG. 6(b). Therefore, the thinner free layer, which is required to sense a smaller media bit with a weaker signal, also has a lower MR and AΔR in the related art CPP scheme. As a result, the related art spin transfer effect problem is increased.
As noted above, further increasing capacity of disk drives requires a small, high-sensitivity MR head that corresponds to the miniaturization of the head size. As head size decreases, the head output signal decreases. Accordingly, the free layer must be more sensitive to the media magnetic field. As discussed in S. Z. Hua et al., Phys. Review B67, 060401 (R) (2003), a high resistance change ΔR can be obtained using the foregoing related BMR concept (i.e., connection of at least two ferromagnetic layers to one another via a nano-contact). A substantially high BMR value can be achieved (e.g., thousands of percent of MR ratio).
The basis of the above-described BMR is disclosed in G. Tatara et al., Phys. Review Letters, Vol. 83, 2030 (1999), based on the thin domain wall between the two adjacent ferromagnetic layers that are antiparallel to each other.
In the related art BMR head, a key factor is the magnetic domain structure. Its configuration control and stability during the read process are extremely important for high-out put signal t. Further, for proper use of the BMR head, it is necessary to stabilize the free layer against thermal agitation and spin transfer effect and make it mono-domain.
Stabilization of the free layer in the related art has been done in the case of CPP-GMR, via an in-stack bias. This configuration is disclosed in U.S. Patent Publication No. 2004/0008454. In this related art in-stack bias, a decoupling layer is formed as a spacer above the free layer. The decoupling layer is made of a continuous conductive film having a thickness of 1 nm to 2 nm. The film may be made of a metal such as Cr, Ta or Cu.
Additionally, Japanese Patent Application Publication No. 10-229013 discloses a magneto-resistive effect element with an in-stack bias. More specifically, a bias film having a structure such that it can stabilize the free layer in mono-domain magnetic structure.